1. Field of the Invention
The present invention relates to a method of manufacturing a radiation resistant semiconductor device used in, e.g., a space satellite, or a nuclear power plant.
2. Description of the Related Art
It is known that when an ionizing radiation such as an x-ray, a Y-ray, or a high-energy charged particle is radiated onto a semiconductor device, a large amount of electron-hole pairs are created in an oxide film. It is well known that a drift velocity of electrons in an oxide film is higher than that of holes. For this reason, the electrons of the electron-hole pairs generated in an oxide film disappear faster than the holes, and the many holes are left, and some portion of these holes are trapped in the oxide film. Therefore a p-type substrate under the oxide film can be inverted into an n-type one due to these trapped holes. As a consequence, when the positive charges (holes) are trapped in a field oxide film, a leakage current between circuit elements is increased. When the positive charges are trapped in a gate oxide film, a threshold voltage of the transistor is changed. Moreover, in a small-geometry LDD transistor, when many positive charges are trapped in the oxide film formed on side walls of a gate electrode, a strong accumulation of electrons otherwise in a low-electron concentration n.sup.- -type region occurs. Therefore, the transistor cannot be operated as an LDD transistor.
As conventional techniques for overcoming the above problem, a guard-band structure is employed to the field oxide film, the field oxide film is formed at a low temperature, or the field oxide film is made of two layers. In addition, it is known that a decrease in thickness and a treatment at a low temperature are effective to hardening of an oxide film.
However, in a semiconductor device having the guard-band structure, since the circuit elements must be arranged in consideration of radiation resistance in a mask design, the design becomes difficult. When the field oxide film is formed at a low temperature, a long time is required for growing the oxide film. Therefore, this method is not practical in the manufacture. In addition, when the oxide film is formed to have two layers, an interface between the two layers disadvantageously disappears by annealing after formation of the oxide film.
As for the oxide film on the side wall of the gate electrode of the LDD transistor, a technique for enhancing radiation resistance has never been developed.